A bounding surface hysteretic water retention model for deformable soils

The paper presents a model that takes into account the influence of void ratio and hydraulic hysteresis on soil water retention. The model is based on the definition of two bounding surfaces, i.e. a main drying surface and a main wetting surface, that delimit the region of admissible soil states in the space of degree of saturation, suction and void ratio. An auxiliary variable, named scaled suction, is introduced to combine the effects of suction and void ratio into a single quantity, so that the main surfaces are recast as curves in the plane of degree of saturation and scaled suction. An increase of scaled suction corresponds to drying of the soil while a decrease of scaled suction corresponds to wetting of the soil; hence the drying/wetting behaviour is governed by changes of both suction and void ratio. The model assumes that the derivative of degree of saturation with respect to scaled suction depends on the distance of the current soil state from the main curves, which ensures a smooth transition of the drying and wetting paths towards their respective main curves. One advantage of the proposed model is that all wetting and drying paths can be integrated in a closed form and are described by means of two explicit equations (one for drying and one for wetting) with different constants of integration. The model requires seven parameters whose values can be obtained from as little as a single drying-wetting test. Predictions are validated against two different data sets published in the literature, showing a good ability of the model to capture the behaviour observed during laboratory tests on fine grained soils.